Photometric apparatus giving readings invariant with azimuth on polarizing samples



Mud! 3, 9 E. I. STEARNS, JR. ET AL 2,438,422

PHDTOIETRIC APPARATUS GIVING READINGS INYARIANT WITH AZIIUTH 0N PQLARIZING SAIPLES Filed Juno 1, 1944 '7 Sheets-Sheet 1 ATTORNEY h 3, 1948- E. I. STEARNS, JR., ET AL PHOTOUETRIC APPARATUS GIVING READINGS INVARIANT WITH AZINUTH 0N POLARIZING SAMPLES Filed June 1-, 1944 7 Sheets-Sheet 2 ATTORNEY r h 1948. a. I. STEARNS, JR, l-rr AL 2,433,422 PHOTOMBTRIC APPARATUS GIVING READINGS INVARIANT WITH AZIIUTH 0N POLARIZING SAIPLES Filed Juno 1, 1944 7 Sheets-Shut 3 mvzm'ons rwmv Jill/PM an.

era/Per 4 e00 a ATTORNEY March 23, 1948- E.- STEARNS, JR. El AL 2,438,422

PHO'I'OIBTRIC APPARATUS exvme READINGS xuwmnn'r \u'ra AZIIIUTH on romaxzme sum-:s

Filed June ,1. 1944 7 Shuts-Shut 4 43 K. .'.i. ql ,2 i. 49

ATTORNEY March 23, 1948. I, STEARNS, JR., ET AL 2,438,422

PHoToIEmIc APPARATUS GIVING nmnmss myARIAnT WITH AzIuuTR on POLABIZING SAMPLES Filed Juno 1. 1944 7 Sheets-Sheet 5 INVENTORS ffi/V/IYA 571 41960: J4.

GIOPG'Z' l 806,

ATTORNEY March 3, 1943- E. I. STEARNS, JR., El Al. 2,438,422

PHOTQIBTRIC APPARATUS GIVING READINGS INVARIANT "1TH AZIIUTH ON POLARIZING SAIPLES Filed June 1, 1944 7 Shoots-Sheet 6 INVENTOR S idly/IV JI'I'AMVKZJIP.

Efdiil'l. 500,

ATTORNEY Patented Mar. 23, 1948 "APPARATUS GIVING BEAD- ARIAN I AZIMU TH N POLARIZING SAMPLES Edwin I. Stearns, In, North l'lainfleld, and

George L. Buc, Orange American N. 1., or: to

Cyanamid Company, New York,

N. Y a corporation of Maine Application June 1, 1944, Serial No. 588,309

1 25 Claims. This invention relates to improved photometric devices involving polarizing elements. A number of photometric devices have been developed and some of them involve polarizing elements. In some cases the polarization is incidental and performs no useful function. Thus, for example, monochromating devices produce varying 'imgrees of polarizationat different wave lengths 101 light in the visible, ultra violet and infrared spectrum. The polarizing may be due to reflection from mirrors or other elements incidental to the photometric apparatus. In most cases the polarization performs no useful functometer, light irom a Van Cittert double monochromator passes through a photometering Rochon prism, then through a Wollaston prism tion but is unavoidable. Ah example of such devices are ordinary photometric spectrophotometers, spectrographs and the like.

Another type of photometric apparatus in which polarization may enter is comparison colorimeters using split comparison fields. An example of such is a Duboscq colorimeter in which polarization results from the mirrors in the instrument. In general, any photometer which uses a mirror will introduce various degrees of polarization.

Some photometric instruments and spectro- 4 photometers use polarization deliberately to effect photometering. A simple, instrument of such a type is a Martens photometer in which light beams from a sample and standard pass through a Wollaston prism which polarizes them at right angles. Polarizers such as a Nicol or a Rochon prism efiect photometering. Thi type of device is also used in the Koenig-Martens spectropho tometer.

' entation of the sample, that is to say, its azimuth with respect to the optical axis of the polarizing elements of the instrument. This variance is known as the azimuth effect and introduces a serious error even in the most highly developed spectrophotometers 0f the polarizing flickering beam type. The first successful commerclal machine of this type is described and claimed in the patent to Orrin Weston Pineo, No. 2,107,836, February 8, 1938. In this spectrophoand finally through a Rochon prism which is rotated at an accurately predetermined rate by means of a, synchronous motor. The two beams from the Wollaston prism which are polarized at right. angles to each other after passing through the rotating Rochon strike samples and. standards of either reflectance or transmission and the total light from the two beams is integrated in an integrating sphere and the integrated light impressed on a phototube which is in the input circuit of a very high gain audio frequency amplifier. The amplified output is then impressed on the armature of a motor, the field of which is supplied with the same current used in driving the synchronous motor and in turn drives the photometering Rochon prism intensity of the flickering beams to cause the in a direction which will result in a change in light from each beamin the integrating sphere to be equal, at which time there is present in the integrating sphere no light fluctuation at flicker frequency and hence no amplifled signal at flicker frequency. The photometering motor may drive an indicating device or an automatic recorder of known design.

The polarized flickering beam spectrophotometer has achieved great success because in its most elaborate modifications curves of photometric absorption or reflectance can be auto-- matically drawn in a very short time. Great as is the advance represented by the polarized flickering beam spectrophotometer over early instruments, in its original form the device had some disadvantages. By that method, the light incldent on the specimen is varying its plane of no larization, so that in many cases an unknown factor of variation is introduced which may vitl- Y ate the results. Further, spurious signals oi the fundamental and harmonic frequencies are produced in the photocell circuit which obscure the vanishing signal at balance and so result in loss of sensitivity in detecting the balance point.

In the patent of O. W. Pineo, No. 2,126,410, August 9, 1938, there are described a number of different flickering means which, when followed by a stationary polarizer, such as a Rochon prism, produced reliable flickering with beams which maintain a constant plane of polarization instead of a plane which rotates with rotation of the flicker prism as in the early Pineo patent referred to above. These means include a rotating retardation plate. such as a half wave plate,

3 Kerr cells impressed with alternating voltage at flicker frequency or magnetostrictive devices. In the improved instrument which is described in the two Pineo patents the reduced sensitivity with polarizing samples is completely overcome and for the first time it becomes possible to obtain reliable and accurate spectrophotometric measurements of polarizing samples. The great advance in sensitivity and accuracy with polarizing samples, however, resulted Only if the orientation of the sample was constant and hence the azimuth of polarization did not change. Conflicting results were obtained if the same sample was measured at different azimuths.

In the patent of O. W. Pineo, No. 2,189,270, February 6, 1940, there is described an improve ment on the instrument using a stationary plane of polarization in which the final Rochon prism is followed either by a quarter wave plate or a Fresnel rhomb. Both of these devices transform plane polarized light into circularly polarized light which is not sensitive to change in azimuth of the sample. The improvement described in the last mentioned Pineo patent, while of definite practical value, was not a complete solution of the azimuth problem because unfortunately a quarter wave plate gives a retardation of exactly a quarter of a Wave length at only one frequency of light. Lights of other frequencies within the visible spectrum are not circularly polarized but are elliptically polarized with ellipses of increasing eccentricity as the frequency of the light varies from that at which the quarter wave plate gives a true retardation of a quarter of a wave length. The elliptically polarized light at other frequencies materially reduces the azimuth effect over plane polarized light but is still not perfect.

The Fresnel rhomb is less sharply selective with respect to wave length and from this standpoint gives a greater degree of azimuth correction. However, as described in the patent, it requires an offset in the light path and presents some mechanical disadvantages as compared to a quarter wave plate.

A somewhat different form of polarizing flickering beam spectrophotometer is one in which the integrating sphere is replaced by a lens which brings together the two divergent beams onto a phototube or other photoelectric device. In order to avoid critical adjustment the image on the photoelectric means may be thrown slightly out of focus or the beams may be focussed on a ground glass or similar diffuser so that the photoelectric means is illuminated by diffuse combined light. In the simple photometric apparatus or Pineo spectrophotometer the azimuth problem is the same.

According to the present invention the azimuth eflect is completely removed at all wave lengths by interposing between the sample and the nearest polarizing element capable of rotating the plane of polarization as a recurrent function of time. Example of such an element is one which is capable of producing a high'intensity magnetic field which may advantageously be in the form of a solenoid with a core of quartz or other suitable material arranged so that the beams pass parallel to the lines of force or a device which exhibits birefringence which can be varied by electrostatic or magnetostrictive action. The rotation of the plane of polarization is the same with either of the above types of devices although they operate by means of somewhat difierent physical properties thus the rotation by means of the magnetic field operates as a result of the socalled Faraday efi'ect whereas the devices exhibiting variable birefringence operate by a varying retardation of polarized light of different orientations. The device is fed with an alternating or pulsating potential at a frequency sufficiently high so that the orientation and state of any polarized light passing therethrough are sym-,

metrically varied throughout at least at a rate higher than that detectable by the photometering devices. In the case of instruments involving visual observation it should be considerably higher than the persistence of vision to obtain complete averaging of polarized light with time. Photographic devices require a number of cycles in a single exposure. Flickering beam spectrophotometers impose a more severe requirement as the alternation must be much more rapid than the flicker frequency, preferably from 5 to 10 times flicker frequency.

The effect of the high frequency electric retardation of the present invention is to vary the state of polarization of each beam with time so that over a period of time corresponding to one photometering cycle asymmetrical polarizing effects are cancelled out.

This invention will be described in greater detail in conjunction with the drawings in which:

Fig. 1 is a perspective view of an indicating spectrophotometer using half wave plate flickering and varying birefringence through magnetostrictive action;

Fig. 2 is a perspective view similar to Fig. 1 in which electrostatic birefringence is used;

Fig. 3 is a perspective view similar to Fig. 2 in which a rotating Kerr cell is used with an instrument having Rochon flickering;

Fig. 4 is a diagrammatic view of a flickering beam spectrophotometer using Rochon flickering and recombination of the two beams instead of an integrating sphere;

Fig. 5 is a diagrammatic elevation of a Koenig- Martens spectrophotometer using a Kerr cell;

Fig. 6 is a perspective diagram of a Hilger spectrograph using a Kerr cell;

Fig. 7 is a perspective diagram of a Duboscq calorimeter using a pair of Kerr cells.

Fig. 8 is a perspective view similar to Fig. 1 in which a quartz core solenoid is used for rotating the plane of polarized light;

Fig. 9 is a perspective view similar to view 8 in which Rochon flickering is used;

Fig. 10 is a diagrammatic view of a spectrophotometer similar to Fig. 4 using a magnetic field for rotating the plane of polarization of the light beams;

Fig. 11 is a diagrammatic elevation of a Koenig- Martens spectrophotometer using a magnetic field for rotation of the plane of polarization of the light;

Fig. 12 is a perspective diagram of a Hilger spectrograph using a magnetic field for rotatin the plane of polarization; and

Fig. 13 is a perspective diagram of a Duboscq calorimeter using a pair of solenoids for rotating the plane of polarization of the beams.

In the modification shown in Fig. 1 a narrow band of light emerges from the exit slit l of the monochromator (not shown) which may be of any conventional design. The beam then passes through a photometering prism 2 carried in a rotatable hollow sleeve 3 which is provided with a pointer 4 moving over a scale 5. The photometering prism which is preferably a Rochon prism serves the purpose of transforming the I maximum to minimum; in opposite phase and the beams finally pass through a birefringent element II which is subjected to varying pressure by a device comprising two magnets 2| having cores of maanetostrictive material which are held in the bottom yoke 23 and top yoke 23 which exert pressure on the element "through the block 2| which is coupled hydraulically to the yoke 23, the coupling being provided with a reservoir 25. The design of the magnetostrictive device is not a part of the present invention being described in detail in the patent'to 0. w. Pineo, No. 2,206,576,

July 2, 1940. The only difference is that insteadof superimposing flicker frequency magnetostrictive impulses on a steady pressure, the magnets are fed with a potential at much higher fre quency. for example, 600-1200 cycles through the transformer 20 and the resonant circuit including the magnets 3i and condenser 21.

The light beams after being subjected to rapid cycles of retardation in the element It pass through conventional decentering lenses l'l into integrating sphere I! through windows l3 where they strike samples and standards which are mounted in the conventional manner back of windows ll. Integrated light from the sphere is imposed on the phototube l 5. In operation when the twobeams of light entering the sphere are of equal intensity and the reflectances of sample and standard are the same there is no change of light in the integrating sphere at flicker frequency. Any change in reflectance with wave length of either sample or standard causes the light to pulsate in the integrating sphere at flicker frequency in phase with the stronger beam and this pulsation is transformed into an electrical signal of flicker frequency by the phototube l5, which signal is then amplified by the high gain audio-frequency amplifier IS, the output of which drives the motor I! which turns the photometering prism 2 through gearing of conventional design. The flelds is of the motor I! and is of the motor {are fed from a source of alternate current of flickerfrequency and are so phased that the motor i1 tends to turn the prism 2 in such a direction as to bring about equalization of light in the integrating sphere. As the motor I! responds only to alternatingcurrent at flicker frequency it will stop as soon as flicker frequency light pulsations cease in theintegrating sphere. The amount by which the photometering prism was turned is a measure of the absorption of the sample at any given wave length and is indicated by pointer 4 on the scale 5. If a recording instrument is desired the motor ll may drive a conventional recorder in the conventional manner.

The operation of the spectrophotometer is the same as that of one which is not provided with the high frequency magnetostrictively actuated element Iii, the only difference being that the azimuth effect is eliminated with polarizing samples. The device being compact, the new element is stationary and can, therefore, be incorporated in the existing spectrophotometer at moderate cost without the necessity of rebuilding the asses whole machine.

. g a r is an important practical advantage particularly with existing spectrophotometers using half wave plate flickering. On the other hand, the magnetostrictively actuated birefringent element of the present invention does not obviate the disadvantage of half wave plate flickering, namely, smaller range. It perform, therefore, only one function, that of the azimuth effect elimination.

Fig. 2 represents a spectrophotometer using half wave plate flickering and a stationary Kerr cell 3| provided with plates 33 oriented at 45 with respect to thestationary Rochon prism 3. The cell may be fllledwith any suitable liquid ex-. hibiting electric birefringence such as a nitrobenzene which is effective where its slight color is not objectionable. The Kerr cell is-supplied in conventional manner with a direct current voltage as indicated on the drawing and on this is superimposed a high frequency alternating potential from transformer 33. The frequency of this alternating current is much higher than flicker frequency, for example, from 600to 1200 cycles per second. The operation of the device is exactly the same optically as that of Fig. l, the rapidly oscillating electric retardation producing pre-' cisely the same opticaieifect as the magnetostrictive retardation.

In Fig. 3 there is shown a spectrophotometer using Rochon flickering in which the Kerrjiell is mounted on the Riochon prism and rotates with it to preserve the orientation of the plates of the Kerr cell with respect to planes of polarization of the beams. This requires the use of slip rings 34 which are supplied with electricity through the brushes 35. D. C. voltageis continuously applied to the plates 32 and on it is superimposed the high frequency alternating voltage from the transformer 33 as described in connection with Fig. 2.

Fig. 4 is a ati'c view of the optics of a polarized light flickering beam spectrophotometer using combined images instead of an integrating sphere. The same parts bear the same reference numbers. The light issuing from themonochromator slit I is imaged on a slit 33 by the lens 64. The beam through a photometering Rochon prism I mounted in a rotatable sleeve 3 which carried a pointer 4 moving over a scale 5. After leaving the slit 33 the beam passes through a Wollastonprism. 3 which splits it into two divergent beams plane polarized at right angles to each other. These beams then pass through a Rochon flickering prism rotated by the motor 9. On the Rochon prism there is mounted 2. Kerr cell 3| with plates 32 oriented at 45' with respect to the axis of the prism. The plates are supplied in the conventional manner with direct lllmnlnates the phototube II, the output of which can be amplified to eifect photometer-mg in the conventional manner as shown in Figs. 1 to 3;

The operation of the spectrophotometer is the sameasthatshowninFigs. 1to3exceptthatit is satisfactory only for transmission samples. The elimination of the azimuth effect and the improved sensitivity of Rochon flickering with polarizing samples is the same as in Fig. 3.

' pass through a bi-prism ly averaged with time by Fig. illustrates a split field type of spectrophotome er, namely, a Koenig-Martens spectrophotometer in which light from a sample and standard is monochromated and photometered. The view is a horizontal elevation and shows the beam from one of the two samples ll. The beam passes through a stationary Kerr cell 31 provided with plates 32 fed with alternating current from a transformer 33 which is superimposed on a steady D. C. The cell may be filled with any suitable liquid such in connection with Fig. 3. The light after passing through the Kerr. cell is collimated by the lens 43 and passes through arotatable dispersing prism 48. Thespectrafromthe two beams then pass through a which is at rightangles to the plane of the figure and is at. 45 to the plates of the Kerr cells. From the Wollaston prism which divides each' beam into two beams plane polarized at right angles to each other, the beams 49 and lens 44 which on a slit 45. The narrow band of light from the slit passes through a photometering polarizer 42 which can be turned to makethe two fields from the sample and standard beams equal in brightness.

Fig. 5 is a diagram of the optics only, the photometering Rochon prism is being mounted in the conventional manner. l

Non-uniform response from polarizing samples and standards at different azimuths is completethe difierent retardations in the Kerr cell. The frequency of the alternating potential being well above the persistence of vision. The usefulness of the instrument is therefore extended to polarizing samples.

Fig. 6 is a diagram, partly in perspective, of a Hilger spectrograph. Light from a source 41 which may be, for example, ultra violet light, passes through the slit 53 collimating lens 60 and sample 62 onto the totally reflecting prism Si inside the body of the spectrograph. Thence the beam passes through a Kerr cell 3|. The construction and operation of the cell are described in connection with Fig. 2. After passing through the Kerr cell the beam goes into the conventional lens 54, dispersing prism 55 with reflecting back, and again through the lens .54 onto the photographic plate holder 56. The diagramis that of a conventional spectrograph except for the Kerr cell 3i. Non-uniform responses from polarizing samples at different-azimuths are completely averaged with time for all wave lengths of the spectrum by means of the Kerr cell. The-frequency of the alternating potentials of the Kerr cell shouldibe suflicient to give a number of complete cycles during the minimum exposure time for photographing spectra. The conventional Hilger spectrograph is thus made useful in the actual measurements of polarizing samples regardless of the azimuth of the sample.

Fig. 7 is a diagram, partly in perspective, of a images the spectra Duboscq colorimeter equipped with a pair of Kerr cells. A source of light 30 illuminates a diffusing screen 28 and the diffused light is reflected by the mirror 29 up through the instrument which includes a sample holder 63, double reflecting prisms 58. lenses 59, ocular 65 and neutral photometering wedge 6|, The only change in the instrument is the provision of a pair of Kerr cells 3| between the mirror 29 and the sample holder and photometering wedge. Non-uniform responses from polarizing samples due to polarization introduced by the mirror 28 are completely as nitrobenzene as described Wollaston prism 46, the axis of eliminated and the ins ument is rendered suitable for measuring polarizing samples with the same accuracy as non-polarizing samples.

In Figs. 5 to '7 Kerr cells have been shown but they can be replaced by magnetostrictive devices as shown in Fig. 1.

Figures 8 to 13 correspond to Figures 2 to 7 except that a solenoid 66 with a quartz core er takes the place of a Kerr cell 3! or, in the case of Figure '7, two solenoids take the place of two Kerr cells. Each solenoid is fed from a high frequency transformer 68 with a condenser 69 across its secondary. An alternating voltage from asuitable high frequency source marked HF feeds the solenoid. The frequency in the case of Figures 8 to 10 may, for example, be from 300 to 600 cycles but is not critical. In the case of Figures 11 and 13 the frequency of the alternating current feeding the solenoid or solenoids should be well above the persistence of vision. I

The operation of the devices using solenoids is exactly the same as that of the devices in the corresponding Figures 2 to 7 and the same averaging out of azimuth effiect is obtained. The parts, other than the solenoid with its core and transformer, are the same in Figures 8 to 13 as in Figures 2 to 7 and, therefore, bear the same reference numerals.

The advantages of the invention are not limited to instruments operating on light in the visible spectrum although these instruments represent the largest field of photometric apparatus. Elimination of the azimuth effect by means of the present invention is just as important with machines using ultra violet light or infrared so long as the light still becomes polarized. For this reason the term light will be used in a broader sense to include ultra violet and infrared. "Similarly where reference is made to illumination it is n-otiintended to limit the term to light in the visible spectrum.

When infrared or ultra violet light is used suitable sources must be employed and also suitable receptors. In the former case for the near infrared they may be phototubes as shown in Figs. 1 to 4 and 8 to 10. Figs. 6 and 12 require only the use of optics of a suitable material and appropriately sensitized film for use in the infrared. In general, a Hilger spectrograph with quartz optics may be used for most measurements in the ultra violet and the near infrared.

Visual observation which is employed with instruments shown in Figs. 5, 7, l1 and 13 must be replaced by photographic recording for the infrared or either photographic recording or fluorescent screen observation for the ultra violet.

We claim:

1. A photometric apparatus comprising in optical alignment means for illuminating a sample support, a photoreceptor system which responds to light fluctuations only up to a predetermined maximum, photometering means operatively connected with the photoreceptor, system.'an essential element of the apparatus being inherently capable of polarizing light to amateriai extent sufilcient to give different respdn'ses from a polarizing sample at various orientations, alternatin current actuated means coaxial with the light path through the apparatus and intermediate the sample support and polarizing element for rotating the plane of polarized light in varying amounts-as a recurrent function of time, and

means for supplying alternating current thereto of a frequency greatly in excess of the maximum frequency of light'fluctuation to which the pho- 'torecegtor system is capable of responding,

' sponses regardless of its orientation on the sample support.

2. A comparison photometric apparatus comprising'in optical alignment illuminating means for supports for a sample and comparison standard, a photoreceptor system which respondsto light fluctuations of a frequency only up to a predetermined maximum, photometering means operatively connected to the photoreceptor system, means for directing light from both sample and standardto the photoreceptor system, an essential element. of the apparatus being inherently capable of polarizing light to a material extent sufllcient to give different responses from a polarizing sample at various orientations, alternating current actuated means coaxial with the li ht path through the apparatus and intermediate the sample support and polarizing element for rotating the plane of polarized light in varying amounts as a recurrent function of time, and means for supplying alternating current thereto of a frequency'greatly in excess of the maximum frequency of light fluctuation to which the photoreceptor system is capable of responding,-

whereby the plane of polarized light is rotated at Y a frequency corresponding to that of the alterhating current supply means and the photoreceptor system receives an .average illumination from polarized light of various orientations so that a polarizing sample gives reproducible responses regardless of its orientation on the sample support.

3. A photometric apparatus comprising, inv optical alignment, 9. photoelectric device positioned to receive integrated light from sample and standard supports, an optical system arranged to control the light received by the sample and standard comprisingin series an angularly movable poiarizing member, a second member having the property of dividing a light beam into two beams which are polarized respectively in planes at right angles to each other, a device between the second member and the sample and standard supports capable of causing each beam 10 sponses regardless of its orientation on the sample support.

4. A photometric apparatus comprising, in op tical alignment, a photoelectric device arranged to receive integrated light from sample and standard supports, an optical system arranged to control theslight received'by said sample and standard comprising in series an angularly movable polarizing member, a second member having the property of dividing a light beam into two beams which are polarized respectively in planes at right angles to each other, an uniformly rotatable third member between the second member and the sample and standard supports capable of causmg each beam to flicker by varying from a minimum to a maximum, the variation being in opposite phase, electric driving means for the first member responsive to alternating current of a predetermined frequency, means for uniformly rotating the beam flickering member at such a speed as to cause the beams to flicker at the same frequency to which the driving means of the first member is responsive, the photoelectric device being coupled to an amplifier capable of amplifying alternating current fluctuations produced by the device in response to fluctuations of light impinging thereon at flicker frequency, means for feeding the alternating current components of the amplifier output to the electric driving means for the polarizing member, the phase of the flicker frequency component of the amplifier output and electric driving means being so adjusted as'to cause the polarizing member to rotate in a direction to produce the same total light from the sample and standard, alternating current actuated means coaxial with the light path through the apparatus intermediate the sample and standard supports and the polarizing element for rotating the plane of polarized light in varying amounts as a recurrent function of time, and means for supplying thereto alternating current of a frequency greatly in excess of flicker frequency, whereby the plane of polarized light is rotated at a frequency corresponding to that of the alternating current, supply means and the photoreceptor system receives an average illumito flicker from a minimum to a maximum, the

flickering of the two beams being in opposite phase, means for amplifying alternating current components of predetermined frequency from said photoelectric device, electrically actuated adlusting means for the polarizing member connected to the output of said amplifying means and responsive to alternating current of said predetermined frequency, an alternating current actuated means coaxial with the light path through the apparatus intermediate the sample and standard supports and the polarizingelement for rotating the plane of polarized light in varying amounts as a recurrent function of time and means for-supplying thereto alternating current of a frequency greatly in excess of the predetermined frequency of the amplifying means, whereby the plan of polarized light is rotated at a frequency corresponding to that of the alternating current supply means and the photoreceptor system receives an average illumination from polarized light of various orientations so that a polarizing sample gives reproducible renation from polarized light of various orientations so that a polarizing sample gives reproducible responses regardless of its orientation on the sample support.

5. A photometric apparatus comprising a. photoelectric device arranged to receive integrated light fromsample and standard supports, an optical system arranged to control the light received by said sample and standard comprising in series an angularly movable polarizing member, a second member having the property of dividing a light beam into two beams which are polarized respectively in planes at right angles to each other and a device between the second member and the sample and standard supports capable of causing each beam to flicker from a minimum to a maximum, the flickering of the two beams in opposite phase, means by'which the polarizing member may be angularly adjusted in accordance with the output of the photoelectric device and means for passing to the flrst polarizing member a beam of substantially monochromatic light, said means being further capable of varying the wave length of said monochromatic light from one end of the spectrum to the other, electric driving means for the angularly movablepolarizing member rell device and capable of amplifying alternating current fluctuation produced by said device in response to fluctuations of light impinging thereon at flicker frequency, means for connecting the amplifier output to the electric driving means, the phase of the flicker frequency of the amplifier output and electric driving means being so adjusted as to cause the polarizing member to rotate in a direction to produce the same total light from the sample and standard, alternating current actuated means coaxial with the light path through the apparatus intermediate the sample and standard supports and the polarizing element for rotating the plane of polarized light in varying amounts as a recurrent function of time, and means for supplying theretfalternating current of a frequency greatly in excess Of flicker frequency, whereby the plane oi polarized light is rotated at a frequency corresponding to that of the alternating current supply means and the photo-receptor system receives an average illumination from polarized light of various orientations so that a polarizing sample gives reproducible responses regardless of its orientation on the sample support.

6. A photometric apparatus comprising, a photoelectric device arranged to receive integrated light from sample and standard supports, an optical system arranged to control the light received by said device comprising in series an angularly movable polarizing member. a second member having the property of dividing a light beam into two beams which are polarized respectively in planes at right angles to each other, a uniformly rotatable third member between the second member and the sample and standard supports capable of causing each beam to flicker by varying from a minimum to a maximum, the variation being in opposite phase, electric driving means for the first member responsive to alternating current of a predetermined frequency, means for uniformly rotating the beam flickering member at such a speed as to cause the beams tofiicker at the same frequency to which the driving means at the first member is responsive, the photoelectric device being coupled to an amplifier capable of amplifying alternating current fluctuations produced by the device in response to fluctuations of light impinging thereon at flicker frequency, means for feeding the alternating current components of the amplifier output to the electric driving means for the polarizing menfber, the phase of the flicker frequency of the amplifier output and electric driving means being so adjusted as to cause the polarizing member to rotate in adirection to produce the same total light from the sample and standard and means for passing to the first polarizing member a beam of substantially monochromatic light, said means being further capable of varying the wave length of said monochromatic light from the end of the spectrum to the other, alternating current actuated means coaxial with the light path through the apparatus intermediate the sample and standard supports and the'polarizing element for rotating the plane of polarized light in varying amounts as a recurrent function of time, and means for supplying thereto alternating current of a frequency greatly in excess of flicker frequency, whereby the plane of'polarized light is rotated at a frequency corresponding to that of the alternating current supply means and the photoreceptor system receives an average illumination from polarized light of various orientations so that a polarizing sample gives reproducibl respouses regardless of its orientation on the sample support.

'7. A photometer having a flicker mechanism including in optical alignment the following elements, a sample support, a photoreceptor system which responds to light fluctuations of a frequency only up to a predetermined maximum, 2. source of light, polarizing means capable of producing two divergent beams polarized at right angles to each other, means for rotating the planes oi polarization of the beams at a predetermined frequency in opposite phases, a fixed polarizing element through which the rotating polarized beams pass before encountering the sample support and an adjustable photometering element the adjustment of which varies the relative intensities of the two polarized beams, a photometric apparatus comprising in optical alignment means for illuminating a sample suport, a photoreceptor system which responds to light fluctuations only up to a predetermined maximum, photometering means operatively connected with the photoreceptor system, an essential element of the apparatus being inherently capable of polarizing light to a material extent sufllcient to give difierent responses from a polarizing sample at various orientations, alternating current actuated means coaxial with the light path through the apparatus and intermediate the sample support and polarizing element for rotating the planeof polarized light in varying amounts as arecurrent function of time, and means for supplying alternating current thereto of a frequency greatly in excess of the maximum frequency of light fluctuation to which the photoreceptor system is capable of responding, whereby the plane of polarized light is rotated at a frequency corresponding to that of the alternating current supply means and the photoreceptor system receives an average illumination from polarized light of various orientations so that a polarizing sample gives reproducible responses regardless of its orientation on the sample support.

8. A photometer comprising, in optical alignment, a source of light, an adjustable photometering polarizer, means for splitting the light into two divergent beams polarized at right angles to each other, means for varying the states of polarization of said beams in opposite phases without substantial variation of the intensities of said beams and 'a fixed polarizing member through which the beams pass, said polarizing member producing two emergent beams which are nonvarying 'in their state of polarization in the sense that the two ellipses symbolizing the states of polarization of the two beams are nonvarying in eccentricity and in orientation but which fixed polarizing member causes the intensities of the non-varying polarized emergent beams to vary in opposite phases at flicker frequency, sample and standard supports receiving light respectively from two beams, a photoreceptor system receiving integrated light from said sample and standard supports, alternating current actuated means coaxial with the light path through the apparatus intermediate the sample and standard supports and the polarizing element for rotating the plane of polarized light in varying amounts asa recurrent function of time, and means for supplying thereto alternating current of a frequency greatly in excess of fiicker frequency, whereby the plane of polarized light is rotated at a frequency corresponding to that of the alternating current supply means and 13 4 the photoreceptor system receives an average illumination from polarized light of various orientations so that a polarizing sample gives reproducible responses regardless of its orientation on the sample support.

9. An apparatus according to claim 1 in which the alternating current actuated means for rotating the plane of polarization comprises a solenoid coil having a transparent core of material producing greater optical rotation than air under the influence of a magnetic field.

10. An apparatus according to claim 2 in which the alternating current actuated means for rotating the plane of polarization comprises a solenoid coilhaving a transparent core of material producing greater optical rotation than air under the influence of a magnetic field.

11. An apparatus according to claim 3 in which the alternating current actuated means for rotating the plane of polarization comprises a solenoid coil having a transparent core of material producing greater optical rotation than air undor the influence of a magnetic field.

12. An apparatus according to claim 4 in which the alternating current actuated means for rotating the plane of polarization comprises a solenoid coil having a transparent core of material producing greater optical rotation than air under the influence of a magnetic field.

13. An apparatus according to claim 7 in which the alternating current actuated means for rotating the plane of polarization comprises a solenoid coil having a transparent core of material producing greater optical rotation than air under the influence of a magnetic field.

14. An apparatus according to claim 8 in which the alternating current actuated means for rotating the plane of polarization comprises a solenoid coid having a transparent core of material producing a greater optical rotation than air under the influence of a magnetic field.

15. An apparatus according to claim 1 in which the alternating current actuated means for rotating the plane of polarization is a magnetostrictive device with a core of birefringent material capable of changing its birefringence under pressure.

16. An apparatus according to claim 2 in which the alternating current actuated means for rotating the plane of polarization is a magnetostrictive device with a core of birefringent mate- 14 rial capable of changing its birefringence under pressure.

17. An apparatus according to claim 3 in which the alternating current actuated means for rotating the plane of polarization is a magnetostrictive device with a core of birefringent material capable of changing its birefringence under pressure.

' 18. An apparatus according to claim 4 in which the alternating current actuated means for rotating the plane of polarization is a magnetostrictive device with a core of birefringent material capable of changing its birefringence under pressure.

19. An apparatus according to claim 6 in which the alternating current actuated means for rotating the plane of polarization is a magnetostrictive device with a core of birefringent material capable of changing its birefringence under pressure.

20. An apparatus according to claim 8 in which the alternat ng current actuated means for rotating the plane of polarization is a magnetostrictive device with a core of birefringent material capable of changing its birefringence under pressure.

21. A device according to claim 1 in which the alternating current actuated means for rotating the plane of polarized light is 2. Kerr cell.

22. A device according to claim 2 in which the alternating current actuated means for rotating the plane of polarizedlight is a Kerr cell.

23. A device according to claim 3 in which the alternating current actuated means for rotating the plane of polarized light is a Kerr cell.

24. A device according to claim 4 in which the alternating current actuated means for rotating the plane of polarized light is a Kerr cell.

25. A device according to claim 8 in which the alternating current actuated means for rotating the plane of polarized light is a Kerr cell.

EDWIN I. STEARNS, JR. GEORGE L. BUC.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number 

